The art of lithographic printing is based on the immiscibility of ink and water. A lithographic printing plate is composed of ink receptive regions, commonly referred to as the “image area,” generated on a hydrophilic surface of a substrate. When the surface of the printing plate is moistened with water and printing ink is applied, revealed portions of the hydrophilic surface retain the water and repel the printing ink, and the oleophilic image area accepts the printing ink and repels the water. The printing ink retained on the oleophilic image area may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the desired surface.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a substrate. Conventional radiation-sensitive coatings include photosensitive components dispersed within an organic polymeric binder. After a portion of the coating is exposed to radiation (commonly referred to as imagewise exposure), the exposed portion becomes either more soluble or less soluble in a developer than an unexposed portion of the coating. A printing plate is generally considered a positive-working plate if, after exposure to radiation, the exposed regions or areas of the radiation-sensitive coating become more soluble and are removed in the developing process to reveal the hydrophilic surface. Conversely, if the exposed regions or areas become less soluble in the developer and the unexposed regions or areas are removed in the developing process, the plate is considered a negative-working plate. In each instance, the undeveloped areas that remain on the plate provide an ink-receptive image, while the revealed regions of the substrate's hydrophilic surface repel ink.
An alternative method for forming an oleophilic image area on a substrate is to imagewise apply an oleophilic composition directly onto the substrate via ink-jet application. A great deal of effort has been expended in attempting to produce an oleophilic ink-jettable composition that has a suitable viscosity and surface tension for effective ink-jet application, low spreadability after application to the substrate, suitable adherence to a substrate after drying and curing, and sufficient durability for commercial printing press use.
For example, European Patent Publication No. 0 503 621 reports a direct method to make lithographic plates by jetting a photocurable composition onto the plate substrate, and then exposing the plate to ultraviolet radiation to harden the image area. An oil-based ink may then be transferred to the image area for printing onto a printing medium. The reported method avoids the need to expose and develop the composition, but still requires a UV curing step to form the image area.
European Patent Application No. 0 591 916 A2 reports a water-based ink having a polymer containing anhydride groups which are thermally cross-linked on the substrate with a hydroxy-functional polymer. This formulation is applied by jetting the formulation (at room temperature) onto a room temperature substrate.
U.S. Pat. No. 4,833,486 discloses a “phase change” type of ink jet composition. The image is produced by jetting the composition at a high temperature, such that the composition solidifies upon contacting the cooler substrate. The ink becomes instantaneously solid rather than remaining a liquid or gel for a period of time prior to drying or curing to form a solid.
Several published applications report compositions that include an oleophilizing compound that reacts with the surface of the substrate. For example, European Published Application No. 1 211 063 reports an ink-jettable composition containing boron, which reacts with the surface of the substrate. European Published Application No. 1 157 827 reports an ink-jet composition including a hydroxyquinoline compound, which reacts with the surface of the substrate. European Published Application No. 1 157 828 reports an ink-jet composition including a 1,3-dicarbonyl group, which reacts with the surface of the substrate European Published Application No. 1 157 826 reports an ink-jet composition including a catechol, a pyrogallol and a salicylic acid, which reacts with the surface of the substrate.
Additional ink-jettable compositions are reported in U.S. Pat. No. 5,688,864, U.S. Pat. No. 5,852,075, U.S. Pat. No. 5,738,013, U.S. Pat. No. 5,688,864, U.S. Pat. No. 5,788,754, U.S. Pat. No. 5,511,477, U.S. Pat. No. 5,312,654, European Patent No. 745,568, Japanese Patent Application No. 08-267902 and European Patent Application Nos. 0 533 168 and 0 003 789.
Despite the significant amount of effort spent developing ink-jettable compositions for printing plate applications, reported compositions tend to suffer from one or more drawbacks. Certain types of compositions tend to spread on the surface of the substrate prior to solidifying the composition by drying or curing. Other compositions lack suitable adherence to substrates after drying or curing. Still other compositions have low resistance to the mechanical stress that plates are subjected to, as well as the chemicals used to clean and treat finished plates. Additionally, many of the polymeric materials commonly employed in ink-jet compositions must be dissolved in organic solvents prior to application using an ink-jet printer. Many organic solvents are being subjected to increased regulation due to environmental considerations. Further, many compositions do not possess a suitable viscosity and/or surface tension to be applied effectively with an ink-jet printer.